Thrombosis is a major cause of morbidity and mortality in Western man, and understanding thrombosis is very relevant to the mission of the NIH. Independent of other risk factors, increasing age is linked to an increase in venous thrombosis and venous thromboembolism. The cell cycle inhibitor protein, p16INK4a, effects an anti- cancer tumor suppressor activity and a pro-aging permanent growth arrest called cellular senescence. Recent work has shown an age-induced increase in p16INK4a expression in certain populations of stem cells and other self-renewing cells causes, in part, the age-associated decline in the regenerative capacity of the associated organ. However, we do not know the relationship between p16INK4a expression, senescence and aging to venous thrombosis. We hypothesize that an aged mouse expressing p16INK4a would have reached cellular senescence within the vasculature;thus, vascular injury might predispose the animal to venous thrombosis and thromboembolism. Our central hypothesis is that an aged-induced increase of p16INK4a expression limits the regenerative capacity of the vascular and hemostasis systems and this increases the risk of venous thrombosis and venous thromboembolism. We will assess the physiological significance of p16INK4a-/- deficiency and transgenic (tg) p16INK4a(tg) over-expressing mice to control C57BL/6 animals with increasing age and its relationship to an increased risk of venous thrombosis and venous thromboembolism. To test this hypothesis, we propose to one, use well-established in vivo vascular injury and stasis models to assess the role of p16INK4a, senescence and aging in venous thromboembolism, and two, further determine the functional roles of isolated blood cells (platelets and monocytes) and endothelial cells using model in vitro assays. The clinical implications of our observations could link aging to p16INK4a expression and the increased risk of venous thrombosis, which would be novel findings. Furthermore, these p16INK4a murine models could provide novel tools to further study aging and venous thrombosis. The information provided by the experiments outlined in this grant proposal will further define the mechanism of how aging functions to predispose individuals to venous thrombosis and thromboembolism, especially related to p16INK4a expression and cellular senescence. While it is evident that much information has been obtained about hemostasis and the role of proteins and cells in promoting and regulating thrombosis, there are many essential molecular and cellular details that remain to be described. Delineating the molecular and cellular properties of p16INK4a expression may ultimately provide new insight into both the pathophysiology and therapies of cardiovascular diseases, and the link to aging and our increased risk for venous thrombosis and venous thromboembolism.